Functional relationships between plant feeding and prey feeding for a zoophytophagous bug

The consumption of plants by the zoophytophagous bug Macrolophus pygmaeus Rambur (Heteroptera : Miridae) in relation to the presence or absence of prey is monitored for 7 days. Tobacco plants enriched with N-15, a stable, nonradioactive isotope of nitrogen, are used for characterization. In the absence of prey, the relationship between time and N-15 content (which equates to plant feeding) is linear, with a constant daily accumulation of N-15; however, if prey are available, the mathematical relationship becomes curvilinear. From day 1 to day 5, feeding on plants is independent of the consumption of prey. The rate of N-15 accumulation decreases in the presence of prey after day 5, whereas the number of prey eaten remains unchanged.